1. Field of the Invention
The present invention relates to an automatic transmission, and more particularly, concerns an automatic transmission with improvements in draining oil for lubricating between a pump drive shaft for driving a pump unit and a bearing hole for supporting the drive shaft.
2. Description of the Related Art
For example, with an oil pump driven by the output rotation of an engine to supply an automatic transmission fluid (hereinafter, referred to as an ATF) to each component of an automatic transmission, the output rotation is transmitted through a sleeve-like pump drive shaft connected to a pump impeller of a torque inverter. The ATF (also referred to as oil or operating oil in the present specification) pumped from the oil pump is supplied for lubrication to a bearing member disposed between the pump drive shaft fitted in a bearing hole of a pump casing for housing the oil pump and the bearing hole, thereby preventing seizing-up and the like of the bearing member during high-velocity rotation.
The operating oil supplied in this way is stored in a fluid reservoir which is formed by sealing the opening end opposite to the bearing member in the bearing hole (adjacent to the torque converter) with an oil seal to lubricate a sliding plane between the bearing member and its surroundings. With such a structure, however, during the operation of the oil pump, the operating oil that is sequentially fed fills the fluid reservoir to increase the pressure, thereby sometimes causing oil leakage from the sealed portion with the oil seal into a converter housing.
Therefore, an oil pump structure has been proposed in which the oil leakage is improved by providing an oil drain passage capable of discharging (draining) the operating oil in the oil reservoir into a mission case to which the pump casing is joined, so that the operating oil can flow smoothly into an oil pan through the oil drain passage and the mission case, thereby restraining an increase in the pressure in the oil reservoir (for example, refer to JP-A-8-121567). The oil pump structure disclosed in the gazette is constructed such that part of the oil drain passage connecting the oil reservoir with the mission case is located higher than the center of the pump drive shaft to thereby prevent the lubricating oil in the oil reservoir from flowing out from the oil drain passage when the oil pump is not operated, thus preventing a shortage of lubricating oil on the sliding plane of the pump drive shaft when the oil pump is driven again.
With the above oil pump structure, however, the operating oil discharged through the oil drain passage into the mission case is spattered around a rotating drum that supports a brake and so on housed in the case, thus having the problem of causing breather oil blow such that the operating oil is stirred by the high speed drum to foam while generating oil drag. The breather oil blow occurs when a breather mechanism is provided for eliminating a difference in the pressure between the inside and the outside of the casing of the automatic transmission by exhausting an air therein to the exterior when the pressure in the casing has become higher than that of the exterior.